The Formatt 4x4" Yellow 3 Glass Filter for Black & White Film is commonly used for a noticeable increase in contrast with black and white films.

If your print seems a bit "muddy", perhaps it needs a slight bump in contrast. Yellow filters are the first step on the contrast ladder to help that print along. Both scenery and skin tones will show more "snap" once this filter is applied. Contrast filters help with the control of gray tones that appear as light and dark tones in a print. 

This filter can also be used to minimize haze during aerial photography.

Especially useful for added contrast between blue sky with clouds and foreground